1. Field of the Invention
The present invention relates to a DC/DC converter.
2. Description of the Related Art
In order to reduce the size and improve the efficiency of a DC/DC converter which generates and outputs a direct current voltage, from an input direct current voltage, whose value of voltage is different from that of the input voltage, it is effective to reduce the switching loss of switching elements by using a soft switching technique, to increase the drive frequency accordingly, and by reducing the size of parts such as a smoothing reactor.
An auxiliary resonance type inverter, that is, an inverter which utilizes the soft switching technique, is widely known. In such an inverter, for example, zero voltage switching, that is, switching of a switching element in a state in which the voltage to be applied to the switching element is kept at zero, is achieved by utilizing the partial resonance of an auxiliary resonance circuit (refer to patent document 1, for example).
An example of a configuration in which the technique according to the above-mentioned patent document 1 has been applied to a DC/DC converter is shown in FIG. 14. A DC/DC converter 101 in FIG. 14 is a step-down type DC/DC converter, in which an output voltage Vout is smaller than an input voltage Vin.
The DC/DC converter 101 shown in FIG. 14 comprises two input terminals 11 and 12 to which a direct current power supply, which is an output power source of the input voltage Vin, is connected, two main switches 1 and 2 connected in series between the input terminals 11 and 12, capacitors C1 and C2 connected in parallel to the main switches 1 and 2, respectively, a smoothing reactor Lo, one end of which is connected to a junction M of the main switches 1 and 2, an output terminal 14 connected to the other end of the smoothing reactor Lo, an output terminal 13 connected commonly together with the input terminal 12, and an output filter capacitor Cout which is connected between the output terminals 13 and 14 and which suppresses variations in the output voltage Vout generated between the output terminals 13 and 14. In addition to the widely known configuration described above, the DC/DC converter 101 further comprises two middle potential generating capacitors (neutral point voltage clamp capacitor) Ca and Cb connected in series between the input terminals 11 and 12, and a resonance reactor Lr, a diode Dr and a switch Sr connected in this order in series between a junction N of the capacitors Ca and Cb and the junction M of the main switches 1 and 2.
In this configuration, the main switches 1 and 2 are each composed of transistors (N-channel MOSFET) S1 and S2 and parasitic diodes D1 and D2 which exist between the drain and the source of the transistors S1 and S2, respectively, and turning-on/off of each transistor S1 and S2 corresponds to turning-on/off of the main switches 1 and 2. On the other hand, as for the input terminals 11 and 12, the input terminal 11 plays the role of the plus terminal on the input side and the input terminal 12 plays the role of the minus terminal on the input side. Similarly, as for the output terminals 13 and 14, the output terminal 13 plays the role of the minus terminal on the output side and the output terminal 14 plays the role of the plus terminal on the output side.
Basically, in this DC/DC converter 101, a period of time (dead time) is provided, during which both the transistors S1 and S2 are maintained off, and the two transistors are turned on/off alternately and at the same time, when the transistor S1 is turned on, the electrical energy from the direct current power supply on the input side is stored in the smoothing reactor Lo and when the transistor S2 is turned on, the electrical energy stored in the smoothing reactor Lo is discharged to a load connected across the output terminals 13 and 14.
In this DC/DC converter 101, for example, when the transistor S2 is turned off and the transistor S1 is turned on (during the period of commutation from S2 to S1), the switch Sr is maintained on for a fixed period of time, the electrical energy is supplied from the above-mentioned junction N to the resonance reactor Lr, and the electrical energy stored therein is used for the resonance operation of the capacitors C1 and C2 and the resonance reactor Lr. After the capacitors C1 and C2 are discharged and charged by the resonance operation, between themselves and the resonance reactor Lr, and when it seems that the voltage between the drain and the source of the transistor S1 falls to zero, the transistor S1 is turned on, and in this way, the zero-voltage turn-on of the transistor S1 can be realized.
According to the soft switching of such an auxiliary resonance type (partial resonance type using an auxiliary resonance circuit), it is not necessary to raise the withstand voltage of each element because the resonance voltage does not exceed the input/output voltage. Moreover, there is an advantage that a PWM control in which the drive period of each transistor S1 and S2 is maintained constant can be carried out and at the same time that designing a noise filter is easy. In other words, in the case of the full resonance instead of a partial resonance, it is necessary to set the off time of the transistors S1 and S2 to a constant value, and change the drive period itself in order to change the duty ratio, therefore, designing an optimum noise filer is complicated, but partial resonance avoids such a problem.
[Patent Document 1]
Japanese Unexamined Patent Publication (Kokai) No. 8-340676
In the configuration shown in FIG. 14, however, it is necessary to newly generate a middle potential (neutral point voltage) which plays the role of the reference of the resonance voltage and the resonance current source, therefore, the two middle potential generating capacitors Ca and Cb are required on the input side, resulting in increase in size and cost of the circuit.
Moreover, in order to securely stabilize the middle potential, it is necessary to additionally provide two balancing resistors Ra and Rb in parallel to the capacitors Ca and Cb, respectively, therefore, the power efficiency is lowered because of the loss at the resistors Ra and Rb.